Polyhydroxycarboxylic acids (aliphatic polyesters), such as polyglycolic acid and polylactic acid, can be degraded or decomposed by microorganisms or enzymes present in natural environments including soils and sea waters and are accordingly noted as biodegradable polymer materials exerting little load to the environments. Further, as polyhydroxycarboxylic acids are degradable and absorbable in vivo, they are also used as polymeric materials for medical use, such as surgical sutures and artificial skins.
Among the polyhydroxycarboxylic acids, polyglycolic acid is excellent in gas-barrier properties inclusive of oxygen gas-barrier property, carbon dioxide gas-barrier property and water vapor barrier property and also excellent in heat resistance and mechanical strength, so that the development for various use thereof as a single material or in a composite form together with another resin material is undertaken in the fields of packaging materials, etc.
A polyhydroxycarboxylic acid can be synthesized by dehydropolycondensation of a hydroxycarboxylic acid, such as glycolic acid (hydroxyacetic acid) or lactic acid (hydroxypropanoic acid), whereas in order to effectively produce a high-molecular weight aliphatic polyester, there has been generally adopted a process of synthesizing a bimolecular cyclic ester of the hydroxycarboxylic acid and subjecting the cyclic ester to ring-opening polymerization. For example, the ring-opening polymerization of glycolide (i.e., bimolecular cyclic ester of glycolic acid) provides polyglycolic acid, and the ring-opening polymerization of lactide (i.e., bimolecular cyclic ester of lactic acid) provides polylactic acid.
In any case, as a starting material for a polyhydroxycarboxylic acid with a high molecular weight and little abnormal linkage content, a hydroxycarboxylic acid is required to have a high purity to some extent, but an industrially available hydroxycarboxylic acid is inevitably accompanied with impurities actually. For example, glycolic acid obtained by carbonylation of formaldehyde in water, in the presence of an organic acid and sulfuric acid as catalysts, contains glycolic acid dimer or oligomer formed by ester-forming dehydrocondensation of glycolic acid and di-glycolic acid (OCCH2COOH)2) that is a dimer formed by ether-forming dehydrocondensation of glycolic acid as major impurities in addition to residues of the catalysts. Then, minor components such as the catalyst residues and ionic impurities can be easily separated and removed industrially by such means as adsorption or ion exchange, but a separate means is required for removal of organic impurities. For example, a patent document, WO92/05138 describes that a 70% technical-grade glycolic acid aqueous typically shows the following composition:
glycolic acid62.4 wt. % glycolic acid dimer8.8 wt. %di-glycolic acid2.2 wt. %methoxyacetic acid2.2 wt. %formic acid0.24 wt. %. 
As general methods for purification or refining by separation of organic materials, unit operations, such as distillation and crystallization, are known. The application of such a purification or refining method to purification of a hydroxycarboxylic acid is, however, accompanied with an inherent difficulty that a hydroxycarboxylic acid readily causes polycondensation under heating. In view of this, distillation involving heating as an essential factor is basically difficult to be adopted. On the other hand, the crystallization from a hydroxycarboxylic acid aqueous solution is essentially a method of applying little thermal load to the hydroxycarboxylic acid but is still accompanied with a difficulty that polycondensation of the hydroxycarboxylic acid is liable to occur when the concentration adopted for efficient crystallization is excessively performed. For this reason, it has been proposed to effect the crystallization from an aqueous for production of a high-purity glycolic acid including the addition of seed crystal (WO 92/05138, mentioned above), whereas the recovery yield of glycolic acid by the crystallization is as extremely low as 6.6%-24% as the process does not include a step of concentrating the aqueous solution while obviating the difficulty of such a concentration step. Such a low yield may be tolerable for production of glycolic acid as a starting material for fine chemical synthesis but is not practical for production of a hydroxycarboxylic acid (glycolic acid) as a starting material for production of polyhydroxycarboxylic acids to be supplied as general-purpose resin products. On the other hand, another patent document, WO 02/22545 discloses an example wherein 327 g of 91 wt. %—lactic acid aqueous solution was subjected to crystallization and centrifugation to obtain 150 g of lactic acid crystal at a yield of 54%, but the technique is a laboratory-scale one and cannot be an industrially feasible process for producing a hydroxycarboxylic acid.